Deposition of thin films



Aug. 26, 1969 K- L. CHOPRA DEPOSITION OF THIN FILMS 7 Sheets-Sheet 1Filed May 7. 1965 PUMP TO VACUUM A E. FIELD A m m0 mH v mL m 2 m FATTORNEYS Aug. 26, 1969 K. L. CHOPRA DEPOSITION OF THIN FILMS 7Sheets-Sheet 2 Filed May '7, 1965 S s A L 60 O NO Om w T G e g m m V M nj m m 2 m 1T C 3 w Am3o 30:55; 855 mm 25o 3OOINVENIOR. KRSTURI L, CHOPRRThickness A FIG. 3

BY 6 H g 3W ATTORNEYS Aug. 26, 1969 K. CHOPRA DEPOSITION 0F THIN FILMSFiled May 7, 1965 7 Sheets-Sheet 3 SILVER ON GLASS lO V/Cm No VoltageThickness (A) FIG. 4

INVENTOR.

K RSTMM L... C'HOPRR RT To RNYS 7 Aug. 26, 1969 Filed May 7, 1965Resisfivify (Relative Value) K. L. CHQPRA 3,463,663

DEPOSITION F THIN FILMS 7 Sheets-Sheet 4 l H l l I l I I00 I20 n40 I60Electric Field (V/Cm) INVENTOR. KRSTUM L. CHO'PRR RT TORNEYS Aug. 26,1969 1.. CHOPRA 3,463,663

DEPOSITION OF THIN FILMS Filed May 7. 1965 7 Sheets-Sheet 5 I 1 l ,1 I ISlLVER ON GLASS O 1 1 I I i l O 20 4O 6O 80 I00 I20 I40 I60 INVENTOR.Electric Field (V/Cm) K STum LCHOPRA AT TORNETS Aug. 26, 1969 CriticalThickness (A) Filed May 7, 1965 K. -L. cHoPRA 3,463,663

DEPOSITION 0F THIN FILMS 7 Sheets-Sheet 6 SILVER ON GLASS No Voltage"-----lO V/Cm Applied O INVENTOR. Temperature C) KASTulu L-QHOPRR[\TTORNEYS Aug. 26, 1969 K. L. CHOPRA DEPOSITION OF THIN FILMS 7Sheets-Sheet 7 Filed May '7, 1965 FIG. 8

INVENTOR. FIG- 9 KRSTUR\ L. CHDPRA BY q WQW ATTORNEYS United StatesPatent U.S. Cl. 117-227 9 Claims ABSTRACT OF THE DISCLOSURE In theformation of thin metal films by vacuum deposition, the maintainence ofan electric field in the plane of the substrate surface inducescoalition of the particles and crystal orientation whereby improvedconductivity at reduced thicknesses is realized.

This invention relates in general to the formation of thin metal filmsby vacuum deposition techniques, and utilizes novel means to control thedeposition of the films to provide several improved characteristics.

Numerous processes are known for forming thin films on various types ofsubstrates. Extremely thin films of metals may be formed on insulatingsubstrates by such techniques as sputtering, pyrolysis of metal formingcompounds, e.g. metal carbonyls, and by vacuum evaporation. Frequentlyit is desirable that the film be electrically conducting, and in allknown processes it is necessary to deposit sufficient material that acontinuous electrical path is formed. This requires that a certainthreshold thickness of film be formed before electrical conductivity isestablished.

The present invention provides substrate-supported con duting filmsthinner than any heretofore obtained by comparable techniques. Inaddition, the process of this invention provides control over thecrystal orientation of vacuum deposited metal films, whereby ultra thinfilms having oriented crystallinity may be produced. The thin filmproducts provided by this invention may be used in such application ascapacitors, wherein the films serve as condenser plates; providescoatings which are extremely thin, and continuous; electrodes orcontacts for semiconductors and other electronic devices; printedcircuits and similar microminaturized electronic applications; andreflective devices such as half mirrors, to name but a few.

The invention is based on the discovery that if an electrical field isapplied in the plane of the substrate, the condensation of metal ontothe substrate is modified in several respects. Fromelectronphotomicrographs of vacuum deposited metal it can be seen thatthe deposit forms as a number of initially unconnected droplets whichgradually build up to the point where they contact, and eventually coverthe substrate surface entirely. Electrical conductivity of the film isestablished when the build up is suflicient to form a continuous pathacross the coalescing droplets. I have discovered that if a DC field isapplied in the plane of the substrate the condensation process ismodified so that the droplets are drawn out and merge when far lessmaterial has been applied. Moreover electron beam diffractionphotographs of these films show a single crystal pattern, whereas filmsformed without the field give a pattern of random orientation.

In general, the thin films of this invention are formed by conventionalvacuum evaporation methods and equipment, with the exception that a pairof spaced electrodes are applied to the substrate surface across which aDC 3,463,663 Patented Aug. 26, 1969 ICC voltage may be applied. Inpractice I prefer a voltage of about volts per centimeter, but as willappear from the "data herein presented, the effect of the appliedvoltage on the film is realized at much lower levels. I have also foundthat certain characteristics of the film may be controlled by varyingthe temperature of the substrate surface. For instance, at lowtemperatures, conductivity is established with films thinner than filmsformed at higher temperatures, but at higher temperatures, a higherdegree of crystal orientation is established than at lower temperatures.

The thin films of this invention may be formed of any of the metalssuitable for vacuum deposition processes, such as gold, silver andcopper on insulating substrates, typically having a specific resistivitygreater than 10* ohm-centimeters. Suitable substrates include glass,mica, quartz, metal oxides, metal halides, and conductive materialshaving an insulating layer of for instance zinc sulfide, bismuth oxide,silicon dioxide, or magnesium oxide.

In practice, the surface of the substrate material to be coated isprovided with a pair of spaced electrodes, which may be vacuum depositedor painted on. After electrical connections have been made to theelectrodes the substrate is placed in the vacuum chamber of conventionalvacuum deposition apparatus, and a potential is applied between theelectrodes so that a voltage gradient exists along the surface at thetime the initial deposit is formed. It is preferable that the voltagesource have a high internal resistance so that large currents sufficientto destroy the film will not flow as the conductivity of the filmincreases.

A description of this invention and the results of its practice aregiven below in greater detail, wherein reference is made to the drawingsin which:

FIG. 1 is a schematic elevation in cross-section of one form ofapparatus for carrying out this invention;

FIG. 2 is a plan view of a substrate surface arranged for the depositionof a film in accordance with this invention;

FIGS. 3 and 4 are graphs showing resistance as a function of thicknessof gold and silver films formed at both no voltage and at 100 v./cm.;

FIG. 5 is a graph showing the resistivity of silver films as a functionof electric field, when formed at 200 C. and 300 C;

FIG. 6 is a graph showing the critical thickness (at which conductivityis established) as: a function of the electric field when formed atvarious temperatures between 100 C. and 320 C;

FIG. 7 is a graph showing the critical thickness of silver films as afunction of temperature when formed at both no voltage and at 100v./cm.;

FIG. 8 is an electronphotomicrograph at 25,000 times enlargement of asilver film 50 A. thick formed on sodium chloride at 300 C. and novoltage;

FIG. 9 is an electronphotomicrograph at 25,000 times enlargement of asilver film 50 A. thick formed on sodium chloride at 300 C. and 100v./cm.

The practice of this invention utilizes conventional vacuum depositionapparatus such as that illustrated in FIG. 1. The material to bedeposited is placed in a basket 14 of a coiled tungsten filamentsupported on power leads 15 and 16 which are mounted in the vacuum table10. The substrate material 20 is provided with a pair of spacedelectrodes 22 and 24 on its surface, to which are attached electricalleads 21 and 23 which also pass through the vacuum table 10, and isplaced on a heating plate 18,

the leads to which extend through the vacuum table to a rheostat controlor variable transformer (not shown). A bell jar 12 supported on thevacuum table 10 provides for the establishment of a vacuum, which isdrawn through the vacuum connection 13.

In a typical operation, a vacuum of about 1O millimeters of mercury isestablished within the chamber. The heating plate 18 is controlled tobring the substrate to the desired temperature, and the desired voltageto cross the substrate is applied to the leads 21 and 23, from highimpedance source, such as a bank of dry cells in series with a 100,000ohm resistor. Power is then applied to the power leads 15 and 16 tocause the filament basket 14 to incandesce and evaporate the metal to beapplied. Typically a movable mechanical shutter (not shown) isinterposed between the basket and the substrate surface to control thedeposition, and is swung to the side after evaporation of the materialto be deposited has commenced.

As the material deposits, it first forms droplets at numerous nucleationcenters and as the nuclei grow the electrostatic attraction appears tocause them to spread out and merge into each other at a much earlierstage in the process than when no field is applied to the substratesurface. If the voltage across the leads 21 and 23 is measured, it willbe seen to be initially high, and then suddenly to drop as conductivityis established. The process is terminated when a film of desiredthickness has been formed.

The effect of establishing a conductive film is illustrated in FIGS. 3and 4 from which it will be seen that a film formed at 100 v./cm. isinitially conducting at a much thinner stage than one formed under novoltage, and has a much lower resistivity at all thickness.

A shown in FIG. 5 a film formed at lower temperatures has lowerresistivity than one formed at higher temperatures, and its relativeimprovement over a film formed at no voltage is greater. Films formed athigher temperatures, however, demonstrate a higher degree of uniformcrystal orientation than those formed at lower temperatures, the lattergiving electron defraction photographs in which the dots characteristicof a single crystal tend to merge into rings.

FIG. 6 similarly shows that as the tempearture is increased the filmthickness at which conductivity is established (critical thickness) isgreater. This may be explained on the basis of the tendency of thedepositing material to accumulate into spheres or globules being greaterat higher temperatures than at lower temperatures. At lower temperaturesthe depositing film appears to be more amenable to being spread out bythe applied voltage.

FIG. 7 similarly demonstrates that as the temperature is increased thecritical thickness increases, however, with an applied voltage of 100v./cm. the critical thickness is less than when no voltage is applied.In addition it will be noted that when no voltage is applied aconducting film of silver can not be formed at a temperature above about320 C., whereas with the application of 100 v./cm. the limitingtemperature is much higher.

The effect of the applied surface voltage gradient is illustrated inFIGS. 8 and 9, the former showing an actual silver film 50 A. thickformed at 300 C. with no voltage. The silver has deposited inunconnected droplets, and the film is not conducting. FIG. 9 shows asimilar film of silver 50 A. thick formed at 300 C. under an appliedvoltage of 100 v./cm. In this case it will be seen that the particleshave merged into one another, and that a definite crystalline patternappears. This film is conducting and an electron diffraction photographsubstantiates its crystalline nature.

In practising this invention, it will be appreciated that theapplication of the electric field is primarily required only during theinitial stage of film formation. After the film has become conductivethe voltage gradient disappears as substantially the entire 4 voltageappears across the current limiting resistor. The deposit which formsunder the influence of the field appears to have something of a seedingeffect on material subsequently deposited, whereby the initialorientation and crystallinity is reflected in the final film. Theinitial deposit thus appears to create a lattice which largely controlsthe crystal structure of the additional material subsequently deposited.

From the foregoing description it will be seen that this invention isessentially one in which metal atoms or molecules are released in avacuum and collected on a substrate surface, and that it is primarilydependent on the conditions prevalent at the substrate surface at thetime of initial deposition. Although the invention has been describedwith reference to the formation of films from evaporated metal vapors,it is conceived of as being applicable to the formation of films byother condensation processes in which the same physical and chemicalfactors are present at the time and place the film substance is formedon the substrate surface. It is accordingly contemplated thatmodifications of the preferred embodiment described herein will readilyoccur to those skilled in the art and familiar with the principlesherein set forth, and that such may be made without departing from thescope of this invention. In particular, it is apparent that alternatingfields may also be employed to achieve certain of the advantageouseffects, particularly the films of increased conductivity. AC fields,however, do not ordinarily give completely oriented crystal structures.The invention is also useful in forming this insulating film, byoxidizing such metal films as silicon or aluminum.

Having thus described my invention, I claim and desire to secure byLetters Patent:

1. The method of forming a thin conductive metal film on an insulatingsubstrate surface, comprising placing the substrate and the metal in achamber, evacuating said chamber to a pressure of less than 10 mm. ofHg, applying an electric field of at least about v./cm. to and in theplane of said substrate surface, heating said substrate to between 100C. and 350 C., and heating said metal sufficiently to cause evaporationthereof, while maintaining said field at said surface, whereby metal iscaused to condense on said surface in the presence of said field.

2. In the process of forming a metal film in which the metal is causedto deposit by a condensation process, on a substrate surface, theimprovement which comprises, coalescing said deposited metal by applyingan electric field to and in the plane of the substrate surface at thetime the metal is initially deposited.

3. A process in accordance with claim 2 wherein the electric field is adirect current electric field and the field is mainained until aconducting film is formed.

4. The process of claim 2 including the further step of simultaneouslyheating the substrate surface and filrn.

5. A process in accordance with claim 2 wherein said electric field hasa value greater than 20* volts per ,centimeter.

6. The process in accordance with claim 2 wherein said electric fieldhas a value of substantially 100 volts per centimeter.

7. A metallized article produced by the process of claim 2 comprising,

a substrate having an electrically insulating surface and a continuousconductive metal film on said surface having a resistivity less thanthat of a like film vacuum deposited without a voltage gradient on thesubstrate surface.

8. The method of forming a thin metal film on a substrate surface whichcomprises,

placing the substrate and the metal to be deposited in a chamber,

evacuating the chamber to a pressure suitable for vacuum deposition,

heating said metal sufliciently to cause the evaporation thereof intosaid vacuum, whereby metal is caused to condense on said substratesurface, and

coalescing said deposited metal by applying an electric field to and inthe plane of said surface while said metal is condensing thereon.

9. The process of claim 8 wherein the substrate surface is heated at thetime the metal is caused to condense thereon.

References Cited UNITED STATES PATENTS 2,999,766 9/1961 Ashworth et al.117-107 X OTHER REFERENCES Sennett I.O.S.A., vol. 40, No. 4, April 1950,pp. 203-211 relied on.

WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R.

